This invention relates generally to phased array radar systems and, more particularly, to an array beamformer which supports the simultaneous formation of plural beams.
In phased array microwave radar systems, it is often required to form two or more simultaneous beams on receive having different weightings. As an example, it may be required to form a sum beam having Taylor weighting and a difference beam having a Bayliss weighting, along a linear array of, illustratively, sixty-four radiating elements.
The beamforming architecture described in this discussion of the prior art is typically used for a single column of a phased array to form two beams on receive. Similar architecture may be used to combine columns into a two dimensional array. One transmit/receive (T/R) module is used per element location. The T/R module typically contains high power amplifiers for transmit, low noise amplifiers for receive, a phase shifter for beam steering, and a level set attenuator. In order to simplify the module design and control functions, it is desirable to use only one phase shifter and one attenuator for both beams.
In accordance with a typical prior art beamforming system, the output of each T/R module on receive is divided in the 1:2 output power coupler to provide equal power levels, which are applied to two distinct, N:1 unequal-split power combiners (beamformers), which form the individual beams. The use of the single phase shifter in the T/R module puts the requirement for phase tracking between the beams on the beamformers. The use of the single attenuator in the T/R module adds the requirement that the beamformers must provide the difference in weighting between the beams. It should be noted that the attenuator in the module provides one degree of amplitude control; the beamformer has only to supply the difference in amplitudes between the beams.
A prior approach for achieving the weighted amplitude distribution required for the N:1 combiners/beamformers is the use of a series of unequal-split 2:1 planar Wilkinson power dividers. The design effort required to implement this approach is time consuming, highly iterative, complex and expensive. Furthermore, the outputs of unequal-split Wilkinson power dividers do not phase track across a frequency band, and, as such, phase compensation networks are required. In the production of such power dividers, accurate control of line width tolerances is needed to minimize amplitude errors, and accurate control of line length tolerances is needed to minimize phase errors.